solar kW question

wind diva
wind diva Registered Users Posts: 1
in Solar Beginners Corner #1
when you buy new solar panels/kits, is the kW listed on the solar system daily consumption/production?
example: is a 5 kW system 5 kW's a day or weekly?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: solar kW question

    Welcome to the forum.

    It would depend on who is selling it and what they are selling. Quite often they use a listing of 'X' kW and mean that is the array size (STC rating of panels * number of panels). How many kW hours it will produce is another matter, as it will be affected by the particular installation.

    You can run the numbers with PV Watts http://www.nrel.gov/rredc/pvwatts/grid.html to get some idea of how much can be produce at your location for a given size array.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: solar kW question
    wind diva wrote: »
    when you buy new solar panels/kits, is the kW listed on the solar system daily consumption/production?
    example: is a 5 kW system 5 kW's a day or weekly?

    The first thing you need to know to keep them honest and educate yourself is that there are two things that count about solar:

    1. Energy. That is what does a certain amount of useful work. Like ten gallons of gasoline will get your car from point A to point B.
    Energy is measured in Kilowatt-hours. More about that later.
    2. Power. That is the rate at which energy is delivered. It is the horsepower of your car engine. Running the engine at X horsepower for Y minutes will also get you from A to B and will use ten gallons of gas in the process.
    Power is measured in Kilowatts.
    AHA! A panel which gives you a power of 1 kW can produce energy at that rate.
    If it is in the sun for one hour, the total energy produced is one Kilowatt-hour, or 1 kWh.

    So, in answer to your question, no, it is not either of these. If the total light hitting your 5 kW panels during the day is equivalent to what they would get in four hours if the sun stayed directly overhead for the whole time, then we say that your solar insolation or sun hours is 4 hours.
    That makes calculating the output of the panels over the course of a full day easy: 5 kW times 4 hours = 20 kWh. That is the same unit that your utility charges you for at somewhere between $.06 per kWh and $.50 per kWh depending on where you live and how much you use.

    Remember that 12 hours of daylight may be only the equivalent of 4 hours of maximum sun. And you will only get that 5kW power on a cold clear day in ideal conditions. Just like few people will get the EPA estimate mileage when they actually drive. But it provides a standardized comparison figure to start from.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #4
    Re: solar kW question

    For an off grid system... They generally mean the output of the off grid inverter (aka 5kWatt inverter). The size of the solar array and battery bank are equally important, but usually not well documented in the glossy.

    For Grid Tied systems--The output of the GT inverter is typically used. For example a 3 kWatt GT inverter will usually have around a 3.5 kW DC rated solar array (California rebates are based on the PTC value of the system output--Not the solar array size by itself).

    But, before you go down that road--First look at your power needs (a Kill-a-Watt type meter is very handy for plug-in AC loads). Then conservation (it is almost always cheaper to conserve power than to generate it).

    Next--Then we can start sizing the system (loads -> battery bank -> solar array -> backup genset).

    Finally--Can start to select components based on system needs (and look at kits--if appropriate).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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